1. Field of the Invention
The invention relates to devices and apparatus for indicating a variable such as the position of a movable member. For example, the invention may be applied to the determination of the position of the dials in a meter such as a water or gas meter or to linear or to rotary encoders.
2. Related Art
The prior art contains a large number of devices and proposals for measuring the relative positions of two members. One proposal, as disclosed in European Patent Application Nos. 240707 and 369493, involves providing a magnet on one of the members and an array of magnetically sensitive resistors on the other, that is to say resistors whose resistance changes in response to the application of a magnetic field thereto. The arrangement is such that as the two members move relative to each other, the magnet moves over the array of resistors. Position is determined by a measuring circuit connected to the array of resistors and operative to detect which resistor has its resistance affected by the magnetic field created by the magnet. The problem with this proposal is that electrical circuitry has to be physically connected to all of the resistors in order to determine the relative positions of the two members. Such circuitry is therefore relatively complex and expensive.
Another proposal, as disclosed in UK Patent Application No. 2016694, employs a magnetostrictive rod effectively operating as an ultrasonic delay line. The rod is attached to one of the movable members and an electrical transmitter coil surrounding the rod and moveable therealong is attached to the other. Application of an alternating pulse to the electrical coil will create an alternating field which will set up an ultrasonic wave in the magnetostrictive rod. Pick up coils are positioned at each end of the rod and connected to an electrical circuit for determining the difference in time of arrival of the ultrasonic wave at the two ends of the rod. This time difference is indicative of the position of the moveable coil with respect to the length of the rod. This proposal again requires electrical circuitry connected to both the transmitter and the pick up coils for determining relative position.
In a further proposal, disclosed in U.S. Pat. No. 4,710,709, one of the relatively moveable members carries two overlapping electrical coils and the other a bar which is moveable axially through the coils and includes a cavity containing a magneto strictive element which is free to vibrate mechanically within the cavity. The first mentioned member also carries means for generating a DC bias magnetic field to which the magnetostrictive element is subjected. As the bar moves through the coils, the strength of the DC bias field on the magnetostrictive element varies and causes the Young's modulus of the element and therefore its resonant frequency to vary also. Change in resonant frequency is therefore a measure of change in position and is detected by applying an interrogating field to the magnetostrictive element by energizing one of the overlapping coils with a signal of appropriate frequency, which may be swept through the range of frequencies at which the magnetostrictive element may resonate. Upon resonance, the magnetostrictive element generates an alternating magnetic field at its resonant frequency, which is detected by the second of the overlapping coils. The problem with this proposal is that the range of displacements which may be detected is limited by the distance through which the magnetostrictive element may move relative to the DC field generating means to produce a detectable variation in resonant frequency.